The intention relates to an electrohydraulic braking system for motor vehicles with a brake cylinder which can be actuated by a brake pedal and a pressureless reservoir for hydraulic fluid, a braking means which is coupled with at least one vehicle wheel and which can be connected with the brake cylinder in order subject the braking means to pressurized hydraulic fluid, a hydraulic pump and a hydraulic pressure accumulator supplied by same for providing pressurized hydraulic fluid which can be supplied to the braking means through a control valve arrangement in a controlled manner in order to subject the braking means to pressurized hydraulic fluid ("brake-by-wire").
In such an electrohydraulic braking system the braking energy required for braking the motor vehicle is provided by an electrically operated braking system In order to brake the vehicle with a minimum delay in the case of an unexpected failure of the electrically operated braking system, the braking system is additionally equipped with a hydraulic auxiliary braking system for the immediate actuation of the brakes.
The range of application for such an electrohydraulic braking system includes antilock control systems, antislip control systems, electronic brake power distribution, as well as driving dynamics control systems.
Due to the fact that in an electrically operated braking system the driver's request is detected by means of sensors at the brake pedal and is supplied to the electronic control unit in the form of electric signals, such systems are also referred as "brake-by-wire" systems. For the redundant hydraulic emergency system a direct connection between the brake pedal and the brakes must be established in a conventional manner via a brake pressure transducer as well as hydraulic lines, which arrangement is also referred to as "push-through". This requires a changeover means so that under normal operating conditions the brake pressure which is generated in the electrical system, and in the event of a defect or a failure of the electric system the brake pressure which is generated in the hydraulic auxiliary system, is transmitted to the brakes.
The control valve arrangement is usually designed as a slide valve so that escapes and leakages are inevitable. Due to this fact, a pressure drop of the hydraulic fluid accommodated in the hydraulic pressure accumulator, in particular in the case of prolonged stop periods, cannot be prevented. In addition, in this known system the components which are arranged down-stream of the hydraulic pressure accumulator (lines, seals, connections, etc.) are permantently subjected to the pressure prevailing in the hydraulic pressure accumulator. This can result in premature material fatigue, leakages and the like.